1. Field of the Invention
The subject matter disclosed herein generally relates to the formation of integrated circuits, and, more particularly, to the formation of field effect transistors comprising at least one strain-creating element formed adjacent a gate electrode thereof.
2. Description of the Related Art
Integrated circuits comprise a large number of individual circuit elements, such as transistors, capacitors and resistors. These elements are connected internally to form complex circuits such as memory devices, logic devices and microprocessors. The performance of integrated circuits can be improved by increasing the number of functional elements in the circuit in order to increase their functionality and/or by increasing the speed of operation of the circuit elements. A reduction of feature sizes allows the formation of a greater number of circuit elements on the same area, hence allowing an extension of the functionality of the circuit, and also reduces signal propagation delays, thus making an increase of the speed of operation of circuit elements possible.
Field effect transistors are used as switching elements in integrated circuits. They provide a means to control a current flowing through a channel region located between a source region and a drain region. The source region and the drain region are highly doped. In N-type transistors, the source and drain regions are doped with an N-type dopant. Conversely, in P-type transistors, the source and drain regions are doped with a P-type dopant. The doping of the channel region is inverse to the doping of the source region and the drain region. The conductivity of the channel region is controlled by a gate voltage applied to a gate electrode formed above the channel region and separated therefrom by a thin insulating layer. Depending on the gate voltage, the channel region may be switched between a conductive “on” state and a substantially non-conductive “off” state.
When reducing the size of field effect transistors, it is important to maintain a high conductivity of the channel region in the “on” state. The conductivity of the channel region in the “on” state depends on the dopant concentration in the channel region, the mobility of the charge carriers, the extension of the channel region in the width direction of the transistor and the distance between the source region and the drain region, which is commonly denoted as “channel length.” While a reduction of the width of the channel region leads to a decrease of the channel conductivity, a reduction of the channel length enhances the channel conductivity. An increase of the charge carrier mobility leads to an increase of the channel conductivity.
As feature sizes are reduced, the extension of the channel region in the width direction is also reduced. A reduction of the channel length entails a plurality of issues associated therewith. First, advanced techniques of photolithography and etching have to be provided in order to reliably and reproducibly create transistors having short channel lengths. Moreover, highly sophisticated dopant profiles, in the vertical direction as well as in the lateral direction, are required in the source region and in the drain region in order to provide a low sheet resistivity and a low contact resistivity in combination with a desired channel controllability.
In view of the problems associated with a further reduction of the channel length, it has been proposed to also enhance the performance of field effect transistors by increasing the charge carrier mobility in the channel region. In principle, at least two approaches may be used to increase the charge carrier mobility.
First, the dopant concentration in the channel region may be reduced. Thus, the probability of scattering events of charge carriers in the channel region is reduced, which leads to an increase of the conductivity of the channel region. Reducing the dopant concentration in the channel region, however, significantly affects the threshold voltage of the transistor device. This makes the reduction of dopant concentration a less attractive approach.
Second, the lattice structure in the channel region may be modified by creating tensile or compressive strain. This leads to a modified mobility of electrons and holes, respectively. Depending on the magnitude of the strain, a compressive strain may significantly increase the mobility of holes in a silicon layer, and may also increase the electron mobility. The mobility of electrons may also be increased by providing a silicon layer having a tensile strain.
A method of forming a field effect transistor wherein the channel region is formed in strained silicon will be described in the following with reference to FIGS. 1a-1d. FIG. 1a shows a schematic cross-sectional view of a semiconductor structure 100 in a first stage of a manufacturing process according to the state of the art. The semiconductor structure 100 comprises a substrate 101. An active region 104 is provided in the substrate 101. A trench isolation structure 102 separates the active region 104 from other elements of the semiconductor structure 100 which are not shown in FIG. 1a. A gate electrode 106, which is separated from the substrate 101 by a gate insulation layer 105, is formed over the substrate 101. The gate electrode 106 is covered by a cap layer 107 and flanked by first sidewall spacers 108, 109. The active region 104, the trench isolation structure 102, the gate electrode 106, the gate insulation layer 105, as well as the first sidewall spacers 108, 109 and the cap layer 107, together form portions of a field effect transistor element 130.
In the formation of the semiconductor structure 100, the substrate 101 is provided and the trench isolation structure 102 is formed by means of methods of photolithography, deposition and/or oxidation techniques known to persons skilled in the art. Then, ions of a dopant material are implanted into the substrate 101 in order to form the active region 104. The type of dopants corresponds to the doping of the channel region of the field effect transistor to be formed. Hence, in the formation of an N-type transistor, ions of a P-type dopant are implanted, whereas ions of an N-type dopant are implanted in the formation of a P-type transistor.
After the formation of the active region 104, an oxidation process is performed to form the gate insulation layer 105. Thereafter, the gate electrode 106 and the cap layer 107 are formed by deposition and photolithography processes known to persons skilled in the art. Subsequently, the first sidewall spacers 108, 109 are formed by depositing a layer of a spacer material and performing an anisotropic etch process, wherein portions of the layer of spacer material over substantially horizontal portions of the semiconductor structure 100 are removed, whereas portions of the layer of spacer material provided on the sidewalls of the gate electrode 106 remain on the substrate 101 and form the first sidewall spacers 108, 109.
A schematic cross-sectional view of the semiconductor structure 100 in a later stage of the manufacturing process according to the state of the art is shown in FIG. 1b. An etch process is performed. The etch process can be an isotropic etch process adapted to selectively remove the material of the substrate 101, leaving the material of the cap layer 107 and the first sidewall spacers 108, 109 substantially intact, for example, a known dry etch process. The cap layer 107 and the first sidewall spacers 108, 109 protect the gate electrode 106, the gate insulation layer 105 and a channel region 140 below the gate electrode 106 from being affected by an etchant used in the etch process.
Portions of the substrate 101 adjacent the gate electrode 106, however, are etched away. Thus, a source side cavity 110 and a drain side cavity 111 are formed adjacent the gate electrode 106. Due to the isotropy of the etch process, portions of the substrate 101 below the first sidewall spacers 108, 109 and, optionally, the gate electrode 106 are removed. Therefore, the cavities 110, 111 may extend below the sidewall spacers 108, 109 and/or the gate electrode 106, the bottom surface 150, 151 of the cavities 110, 111 having a somewhat rounded shape.
After the etch process, the cavities 110, 111 may have a rough surface. If a strain-creating material were deposited over the substrate 101 in order to fill the cavities 110, 111 as described below, unevenness on the bottom surface 150, 151 of the cavities 110, 111 would act as nucleation sites, leading to an undesirable polycrystalline growth of the strain-creating material. Therefore, a process is performed to reduce the roughness of the bottom surface of the cavities.
The roughness reducing process can be a high temperature prebake process wherein the semiconductor structure 100 is exposed to a temperature in a range from about 800-1000° C. for about 30 seconds to about 10 minutes. During the prebake process, the semiconductor structure 100 can be provided in an ambient comprising hydrogen gas which substantially does not react chemically with the materials of the semiconductor structure 100. The high temperature prebake process leads to a diffusion of atoms on the surface of the cavities 110, 111. Due to the diffusion, a material transport may occur which leads to a reduction in the roughness of the surface of the cavities 110, 111.
FIG. 1c shows a schematic cross-sectional view of the semiconductor structure 100 in yet another stage of the manufacturing process. Strain-creating elements 114, 115 are formed adjacent the gate electrode 106. To this end, the cavities 110, 111 are filled with a layer of a strain-creating material. In methods of forming a field effect transistor according to the state of the art, the strain-creating material may comprise silicon germanide. As persons skilled in the art know, silicon germanide is an alloy of silicon (Si) and germanium (Ge). Other materials may be employed as well.
Silicon germanide is a semiconductor material having a greater lattice constant than silicon. When silicon germanide is deposited in the cavities 110, 111, however, the silicon and germanium atoms in the strain-creating elements 114, 115 tend to adapt to the lattice constant of the silicon in the substrate 101. Therefore, the lattice constant of the silicon germanide in the strain-creating elements 114, 115 is smaller than the lattice constant of a bulk silicon germanide crystal. Thus, the material of the strain-creating elements 114, 115 is compressively strained.
The strain-creating elements 114, 115 can be formed by means of selective epitaxial growth. As persons skilled in the art know, selective epitaxial growth is a variant of plasma enhanced chemical vapor deposition wherein parameters of the deposition process are adapted such that material is deposited only on the surface of the substrate 101 in the cavities 110, 111, whereas substantially no material deposition occurs on the surface of the first sidewall spacers 108, 109 and the cap layer 107.
Since the strain-creating elements 114, 115 are compressively strained, they exhibit a force to portions of the substrate 101 in the vicinity of the gate electrode 106, in particular to portions of the substrate 101 in the channel region 140. Therefore, a compressive strain is created in the channel region 140.
FIG. 1d shows a schematic cross-sectional view of the semiconductor structure 100 in yet another stage of the manufacturing process according to the state of the art. After the formation of the strain-creating elements 114, 115, the first sidewall spacers 108, 109 are removed. Additionally, the cap layer 107 may be removed. Thereafter, an extended source region 116 and an extended drain region 117 are formed in portions of the substrate 101 and the strain-creating elements 114, 115 by means of an ion implantation process known to persons skilled in the art. In the ion implantation process, ions of a dopant material are introduced into the substrate 101 and the strain-creating elements 114, 115. In case of the formation of an N-type field effect transistor, ions of an N-type dopant are introduced, wherein ions of a P-type dopant are provided in the formation of a P-type transistor.
Subsequently, second sidewall spacers 118, 119 are formed adjacent the gate electrode 106. Thereafter, a further ion implantation process is performed to form a source region 120 and a drain region 121 by introducing dopant material ions.
Finally, an annealing process may be performed to activate the dopant materials introduced in the formation of the extended source region 116, the extended drain region 117, the source region 120 and the drain region 121.
One problem associated with the above method of forming a field effect transistor according to the state of the art is that a relaxation of strain may occur in the strain-creating elements 114, 115. Therefore, the strain in the strain-creating elements 114, 115 and consequently the strain in the channel region 140 can be reduced. This may lead to a reduced enhancement of the mobility of holes and/or electrons in the channel region.
The present disclosure is directed to various methods that may avoid, or at least reduce, the effects of one or more of the problems identified above.